Many different types of centrifugal separators are known for separating heterogeneous mixtures into components based on specific gravity. A heterogeneous mixture, which may also be referred to as feed material or feed liquid, is injected into a rotating bowl of the separator. The bowl rotates at high speeds and forces particles of the mixture, having a higher specific gravity, to separate from the liquid by sedimentation. As a result, a dense solids cake compresses tightly against the surface of the bowl, and the clarified liquid, or “centrate”, forms radially inward from the solids cake. The bowl may rotate at speeds sufficient to produce forces 20,000 times greater than gravity to separate the solids from the centrate.
The solids accumulate along the wall of the bowl, and the centrate is drained off. Once it is determined that a desired amount of the solids has been accumulated, the separator is placed in a discharge mode. In one such discharge mode, a scraper blade extending the length of the rotating bowl is placed in a scraping position against the separator wall and the bowl is rotated at a low scraping speed. Then, a radial-motion scraper scrapes the solids from the sides of the bowl, and they fall toward a solids collecting outlet. However, such a radial-motion scraper does not effectively remove wet or sticky solids which may have a consistency like that of peanut butter. In such instances, the sticky solids remain stuck on the scraper blades or fall from the wall and then reattach to the blades before reaching the collecting outlet. As a result, the solids recovery yield is reduced and the remaining solids undesirably contaminate the separator.
Still, other separators do not provide a convenient means by which to handle and recover sensitive solids, such as proteins. For example, an operator is commonly used to assist with solids discharge and recovery, using a complicated and costly solids discharge valve. Separators that require such operator intervention often suffer from contamination problems. Furthermore, some separators employ numerous mechanical components to facilitate solids recovery, which can affect separator durability. Such components are usually external to the separator or in the form of add-on equipment that poses both size and compatibility issues. Conventional separators also tend to be difficult to clean or sterilize without significantly increasing maintenance costs.
Sensitive solids may also suffer from degradation during the centrifugation process, thus reducing the usable yield following centrifugation. For example, foam may be generated at any air interface where feed liquid is added to a separator bowl, increasing fluid shear and causing product degradation. Further, exposure to air at the liquid/air interface can increase product oxidation in bio-pharmaceutical fluids.
It is desirable to have a centrifugal separator that can be effectively used with solids that result in sticky accumulations and/or are sensitive to shear forces generated during centrifugation, thereby recovering a higher amount of solids from the centrate. Such solids (e.g. proteins) would have a lower level of degradation and/or oxidation, and thus a higher level of activity. It would also be useful to have a separator that can easily recover such solids without the possibility of external contamination or additional mechanical equipment. Such a separator should also be able to be conveniently cleaned or sterilized-in-place.